A photoelastic modulator (PEM) is an optical device for the modulation of polarisation or phase modulation of light. PEMs exploit the photoelastic effect, whereby an optical element exhibits birefringence—that is, the refractive index of the element is different for different components of polarised light—that is proportional to the amount of strain (and hence deformation) induced in the element.
The modern PEM was invented in 1969 by J. C. Kemp (J. C. Kemp, Journal of the Optical Society of America, 59 (1969) 950-954), whose basic design has been used in most polarisation related spectroscopy ever since. The Kemp design has the advantage that a pure longitudinal standing wave leads to near pure polarisation in the desired direction. FIG. 1 is a view of a typical two block PEM 10 according to the background art, with coordinate system shown. (For convenience, this coordinate system is also referred to below in the context of the present invention.) PEM 10 comprises an optical block 12 with an aperture 14 to transmit the light beam to be modulated, and a driving block 16 affixed thereto. The centre of the aperture 14 is positioned at the mid-point of the block 12 and corresponds to the maximum strain volume of the optical block 12. FIG. 2 is a schematic plot of the strain distribution within the optical block 12 along the X-axis. Antinodes are labelled “A” and nodes are labelled “N”. FIG. 3 is a schematic plot of the transverse perturbation caused by the longitudinal standing wave induced in the optical body 12 of PEM 10. Region 32 corresponds to the nodes “N” of FIG. 2, while region 34 is the region of maximum strain corresponding to the antinodes “A” of FIG. 2.
However, this form of PEM is sensitive to variations in ambient temperature, which poses difficulties in applications outside temperature controlled laboratories. Another PEM, invented by J. C. Canit and J. Badoz (J. C. Canit and J. Badoz, Applied Optics, 22 (1983) 592), was claimed to have very high efficiency and to be less vulnerable to temperature variations. The Canit-Badoz device employs so-called “shear coupling” to achieve high efficiency, which they effect by adhering a thin slab of piezoelectric ceramic transducer to one surface of a rectangular optical element, but this creates new problems. The surface induced longitudinal acoustic standing wave that is the driving force for creating optical birefringence causes considerable unwanted vibrations and acoustic reflections. These unwanted components tend to interfere with each other and degrade the purity of the polarisation modulation. In practice, the suggested use of two piezoelectric transducers—one on the top surface and one on the bottom surface of the PEM body—has been shown to operate with low efficiency, and this design has been mentioned little in the measurement of polarisation.
FIG. 4 is a schematic view of a PEM 40 according to the Canit-Badoz approach. PEM 40 comprises a rectangular block 42 of solid material and a piezoelectric transducer 44 in the form of a flat slab adhered to one of the narrow faces of block 42, to vibrate block 42 in the Y (or transverse) direction and hence establish longitudinal standing waves extending in the X (or longitudinal) direction. The PEM 40 also comprises, adhered to the face opposite transducer 44, either a second, identical piezoelectric transducer or piezoelectric sensor 46 (which would be of comparable dimensions).
Piezoelectric transducer 44 is FIG. 5A is a view of piezoelectric transducer 44 of PEM 40. Transducer 44 has a top electrode 52a, a bottom electrode 52b, and electrical leads 54a, 54b coupled to electrodes 52a, 52b respectively.
Canit and Badoz suggested that the piezoelectric transducer 44 couples its vibration through shear action to the block 42. Although the Canit-Badoz device is effective, no experimental evidence or theoretical study has yet verified the suggested mechanism. Further, the shear coupling create considerable internal reflections together with some resonances at different acoustic frequencies; these perturb the normal longitudinal standing wave. Based on the evidence that a spectrum of acoustical resonance peaks can be obtained by sending signals of different frequencies to excite those resonances and at frequencies of some resonance peaks, suggesting a considerable amount of optical retardation. Although the frequencies are separated from the desired longitudinal frequency, it appears likely that polarisation purity is adversely—if transiently—affected. The acoustical spectrum is relatively easy to measure, but it has proved to be extremely difficult to visualise them as acoustic waves and to analysis their individual components.
A recent attempt to solve the problem of the propagation of unwanted vibration and reflections into the optical block is described in WO 06/079168, which discloses a PEM with good thermal stability while retaining the high transduction efficiency of the Canit-Badoz design. WO 06/079168 employs, in some embodiments, transducers comparable to those of the Canit-Badoz design (cf. FIG. 5A). FIG. 5B is a view of piezoelectric sensor 56 of WO 06/079168 (see FIG. 13 thereof), with electrical leads 58a, 58b, and which is narrower than the corresponding transducer in order to minimise sensitivity to longitudinal perturbations.
However, owing to the inherent limitations of the Canit-Badoz driving scheme, the PEM of WO 06/079168 also has limitations such that, for example, a two block structure remains preferable, with an optical block of smaller cross-section employed to minimise the propagation of unwanted vibration. Where a crystalline material such as calcium fluoride is used for that optical block, it is typically necessary to precisely cut the crystal along two specific crystal directions or use different material for the construction of the driver block; in the latter arrangement, the advantage of high temperature stability is lost.